In the UMTS (Universal Mobile Telecommunication System) network, the specifications of long term evolution (LTE) have been drafted for the purpose of further increasing high speed data rates, providing lower delays and so on (see non-patent literature 1). The specifications of LTE-advanced (also referred to as LTE “Rel. 10,” “Rel. 11” or “Rel. 12”) have been drafted for the purpose of further broadbandization and speed-up from LTE (also referred to as “LTE Rel. 8”), and a successor system (LTE of Rel. 13 and later versions) is also under study.
Carrier aggregation (CA) to integrate multiple component carriers (CC) is introduced in LTE Rel. 10/11 in order to achieve broadbandization. Each CC is configured with the system bandwidth of LTE Rel. 8 as one unit. In addition, in CA, multiple CCs under the same radio base station (eNB: eNodeB) are configured in a user terminal (UE: User Equipment).
On the other hand, in LTE Rel. 12, dual connectivity (DC), in which multiple cell groups (CG) formed by different radio base stations are configured in a user terminal, is also introduced. Each cell group consists of at least one cell (CC). In DC, since multiple CCs of different radio base stations are integrated, DC is also referred to as “inter-eNB CA.”
In above-mentioned LTE Rel. 8 to 12, the transmission time intervals (TTIs) that are applied to DL transmission and UL transmission between radio base stations and user terminals are configured to one ms and controlled. Transmission time intervals are also referred to as “communication time intervals,” and a TTI in LTE systems (Rel. 8 to 12) is also referred to as a “subframe duration.”